perf: Skip request-body compression for already-compressed content types (#987)

### What

`_prepare_request_call` now checks the request's `Content-Type` before
compressing the body and skips compression for media types that already
carry their own compression:

- any `image/*`, `audio/*`, or `video/*` type,
- archives (`application/zip`, `application/gzip`,
`application/x-7z-compressed`, rar, bzip2, xz, zstd, jar),
- web fonts (`font/woff`, `font/woff2`).

Types with a `+json`/`+xml` structured suffix stay compressible, so
`image/svg+xml` is not caught by the `image/` prefix. The media type is
normalized before matching (parameters stripped, lowercased, trimmed).
When compression is skipped, any caller-supplied `Content-Encoding`
header is dropped as well — the body goes out verbatim, so the header
would otherwise misdescribe it.

### Why

The client compressed every request body unconditionally. For
already-compressed payloads that burns CPU and holds a second full copy
of the body in memory, while typically producing output slightly
*larger* than the input. Key-value store records — screenshots, video,
archives — are exactly this case.

End-to-end `set_record` with a 200 MB incompressible payload:

| Content type | Peak RSS | Wall / CPU |
|---|---|---|
| `application/octet-stream` (unchanged) | 604 MB - 3.02x | 7.8 s / 6.9
s user |
| `image/png` (new path) | 403 MB - 2.02x | 1.05 s / 0.40 s user |

`application/octet-stream` is deliberately not on the list: it is the
catch-all for unknown binary, which may well be uncompressed data. It is
also the encoder's default when no `content_type` is passed, so the
optimization only applies when the caller sets an accurate content type
— documented in the compression concept page.

The remaining 2x comes from handing a `bytes` body to impit's
`content=`, tracked separately in #972.

### Notes

This overlaps with #934 (skip compression for small payloads) — both add
a condition to the same block in `_prepare_request_call`. Whichever
lands second needs the two conditions combined.

*✍️ Drafted by Claude Code*